Method and Apparatuses for Reducing Feedback Overhead

ABSTRACT

The embodiments herein relate to method performed by a radio network node, a network node, a method performed by a UE and a UE for reducing feedback overhead. The method perform by the UE comprises at least: decomposing each entry corresponding to a (i,j)-th combining coefficient of a precoder matrix into at least two coefficients; quantizing, separately, each of said at least two coefficients with a least one bit, and reporting information related to at least one phase value or at least one amplitude value or at least one phase value and an amplitude value of said quantized coefficient.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage application, filed under 35 U.S.C.§ 371, of International Patent Application No. PCT/EP2018/081125 filedon Nov. 13, 2018, which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present disclosure relates to the field of wireless communications,and in particular to methods and apparatuses for reducing feedbackoverhead (e.g. CSI feedback) by employing efficient amplitude and phasequantization and reporting of coefficients in a communications network.

BACKGROUND

Beamforming is a crucial part of the third Generation PartnershipProject (3GPP) Release (Rel.) 15 which define a New Radio (NR) accesstechnology that enables a radio base station (also denote herein gNB)and a User Equipment (UE) to establish and adapt communication linksusing spatially precoded pilot signals. Important information in 5G toimprove communication links and to efficiently adapt the beamformingtechnique is feedback reported by a gNB and/or a UE regarding ChannelState Information or CSI feedback reporting.

In Rel.-15 Type-II CSI reporting, it is assumed that dual-stageprecoding is performed in the frequency domain on a per subband basis,i.e., a single precoder is calculated for a group of adjacent PhysicalResource Blocks (PRBs), referred to as a ‘subband’. The Rel.-15 Type-IIdual-stage precoder comprises two components: A first-stage precoderdenoted F₁ that is identical for all subbands and which contains theselected entries/beams selected from a Discrete Fourier Transform basedcodebook (DFT-based codebook), and a second stage precoder denoted F₂which contains the subband-dependent beam-combining coefficients of allsubbands.

The feedback overhead for reporting the beam-combining coefficientsincreases approximately linearly with the number of subbands, and itbecomes considerably large for large numbers of subbands. To overcomethe high feedback overhead of the Rel.-15 Type-II CSI reporting, it hasbeen decided in 3GPP-Radio Access Network (RAN) standardization meeting,3GPP RAN#81 [1] to study feedback compression schemes for the secondstage precoder F₂. In several contributions [2]-[7], it has beendemonstrated that the number of beam-combining coefficients in F₂ may bedrastically reduced when transforming F₂ using a small set of DFT orDiscrete Cosine Transform (DCT) basis vectors into the delay domain.

Rel.-15 Dual-Stage Precodinq and CSI Reporting:

Assuming a rank-R transmission and a dual-polarized antenna array at thegNB with configuration (N₁, N₂, 2), the Rel.-15 double-stage precoderdisclosed in [8] for the s-th subband and r-th transmission layer isgiven by

$\begin{matrix}\begin{matrix}{{{F^{(r)}(s)} = {F_{1}^{(r)}{f_{2}^{(r)}(s)}}},} \\{= {F_{1}^{(r)}F_{A}{{\hat{f}}_{2}^{(r)}(s)}}}\end{matrix} & (1)\end{matrix}$

where the precoder matrix F^((r))(s) has 2N₁N₂ rows corresponding to thenumber of ports, and S columns for the reporting subbands/PRBs. Thematrix F₁ ^((r))∈

^(PN) ¹ ^(N) ² ^(×2L) is the wideband first-stage precoder containing 2Lspatial beams for both polarizations, which are identical for all Ssubbands, and F_(A) is a diagonal matrix containing 2L widebandamplitudes associated with the 2L spatial beams, and f₂ ^((r))(s) is thesecond-stage precoder containing 2L subband (subband amplitude andphase) complex frequency-domain combining-coefficients associated withthe 2L spatial beams for the s-th subband.

According to [8], the reporting and quantization of wideband amplitudematrix F_(A) and subband combining coefficients in f₂ ^((r))(s) arequantized and reported as follows:

Quantization and Reporting of Wideband Amplitudes of Matrix F_(A)

-   -   The wideband amplitude corresponding to the strongest beam which        has an amplitude value of 1 is not reported. The wideband        amplitude values associated with the remaining 2L−1 beams are        reported by quantizing each amplitude value with 3 bits.

Quantization and reporting of amplitude and phase values of subbandprecoder {circumflex over (f)}₂ ^((r))(s):

-   -   The amplitudes and phase values of the coefficients associated        with the first leading beam are not reported (they are assumed        to be equal to 1 and 0).    -   For each subband, the amplitudes of the B coefficients        associated with the first B−1 leading beams (other than the        first leading beam) are quantized with 1 bit−[√{square root over        (0.5)} 1].    -   The amplitude values of the remaining 2L−B beams are not        reported (they are assumed to be equal to 1).    -   For each subband, the phase values of the B−1 coefficients        associated with the first B−1 leading beams (other than the        first leading beam) are quantized with 3 bits.    -   The phase values of the remaining 2L−B beams are quantized with        2 bits.    -   The number of leading beams for which the subband amplitude is        reported is given by B=4, 4 or 6 when the total number of        configured spatial beams L=2, 3, or 4, respectively.

Space-Delay Precoder:

Collecting the precoders F^((r))(s) for all S subbands in a matrixF^((r)), we obtain [2, 3, 7]

$\begin{matrix}\begin{matrix}{{F^{(r)} = {F_{1}^{(r)}\left\lbrack {{f_{2}^{(r)}(0)}\mspace{14mu}\ldots\mspace{14mu}{f_{2}^{(r)}(s)}\mspace{14mu}\ldots\mspace{14mu}{f_{2}^{(r)}\left( {S - 1} \right)}} \right\rbrack}},} \\{= {F_{1}^{(r)}{F_{2}^{(r)}.}}}\end{matrix} & (2)\end{matrix}$

Then the second-stage precoder F₂ ^((r)) can be written as:

F₂ ^((r))=[f_(2,1) ^((r)) . . . f_(2,u) ^((r)) . . . f_(2,2U)^((r))]^(T), whose u-th row contains the complex combining-coefficientsassociated with the u-th beam over S subbands,

f _(2,u) ^((r)) t=[f _(2,u) ^((r))(0) . . . f _(2,u) ^((r))(s) . . . f_(2,u) ^((r))(S−1)]^(T)∈

^(S×1).

By considering a transformation on the subband precoder F₂ ^((r)), theoverall precoder may be written as

F ^((r)) =F ₁ ^((r)) W ₂ ^((r)) K _(F) ^((r)) ^(T) ,  (3)

where matrix W₂ ^((r))∈

^(2L×V) contains the complex-combining coefficients and the matrix K_(F)^((r))∈

^(V×S) is composed of a number of basis vectors used to perform acompression in the frequency domain. In general, when V<S a compressionof the combining coefficients F₂ ^((r)) is achieved. Each complexcoefficient in W₂ ^((r)) in (3) is associated with a specific delay (inthe transformed domain) as each DFT/DCT basis vector models a linearphase increase over the subbands.

The number of spatial beams and indices of spatial beams may bedifferent, identical, partially identical or non-identical over thetransmission layers.

In addition, with respect to the spatial beams, the delays may bepartially identical or non-identical over the beam. Due to differentspatial beam configuration over the layers, the delay configuration mayvary over the layers as well. Therefore, multiple configurations of thebeam and delay configurations are possible. However, the spatial beamand delay configuration of the space-delay precoder shall be alignedwith the physical structure of the radio channel. The radio channel iscomprised of a number of clusters of scatterers associated withrespective delays (see channel cluster #1, delay #1, . . . , channelcluster #3, delay #2 in FIG. 1).

In the example of FIG. 1, each transmit spatial beam of the gNB (beam#1,beam#2, beam#3 and beam#4 in this example) is associated with a singleor few channel clusters with corresponding delays. Beam #1 is associatedwith cluster #1 and delay #1. Beam #2 and Beam #3 are associated withthe direct Line Of Sight (LOS) channel component and with cluster #3 anddelay #3. Beam #4 is associated with cluster #2 and delay #2. As shown,the delays of cluster #1 and cluster #2 are different and longer thanthe delay of cluster #3 (closest to the UE).

In order to capture a significant portion of the energy of the radiochannel at the UE, the spatial DFT/DCT beams of the first stage precoderneed to point in the direction of the channel clusters. In a typicalchannel setting the clusters are uniformly distributed around the gNB,and each transmit spatial beam is associated with a single or fewneighbored clusters. Moreover, due to the uniform distribution of thechannel clusters, each cluster is associated with a different delay. Thenumber of clusters to which each spatial beam is associated with dependsmainly on the beam width (which is related to the aperture size of theantenna array at the gNB). The larger the beam width (i.e., the smallerthe aperture size of the antenna array) the more channel clusters areassociated with the spatial beam. Therefore, the delay configuration(number of delays per beam and the values of the delays) of each spatialbeam depends on the channel cluster(s) to which the spatial beam isassociated with.

For the transformed precoder, each spatial beam is associated witheither a single or a small set of delays. The transmit beams are hence“delayed” by specific delay(s) before transmission. The delays need tobe selected in such a way that all 2L beams are coherently combined atthe UE. Note again that each delay is represented by entries of aDFT/DCT vector which models a linear phase increase over the subbands.

The selection of the delays for a spatial beam is therefore identical toa selection of DFT/DCT vectors. Due to the delay distribution of thechannel clusters, it should be understood that the delays associatedwith one spatial beam might be different to the delays associated withanother spatial beam. Similarly, the delay configuration (number ofdelays and the delay values) may be different for different beams.Different delay configurations are discussed in detail in applicant'sdocuments [2], [3], [7]. Note that document [7] has not yet beenpublished when the present application is filed and hence [7] is not tobe considered prior art for the subject matter of the claims andteachings of the present application.

When different DFT/DCT vectors are used per spatial beam, a matrixcontaining all selected DFT/DCT vectors of all configured spatial beamsmay be used to form a common matrix for the transformation. The commontransformation matrix contains all the selected DFT/DCT vectors of allbeams. When such a matrix is used for the transformation, the combiningcoefficients associated with a beam contain few non-zeros coefficientsonly for the DFT/DCT vectors it is associated with and zeros elsewhere.Therefore, the complex combining coefficients in matrix W₂ ^((r)) in (3)may contain a large number of values which are close to zero.

-   3GPP DRAFT; R-1812242, 3^(RD) GENERATION PARTNERSHIP PROJECT (3GPP),    MOBILE COMPETENCE CENTER; 650, ROUTES DES LUCIOLES; F-06921    SOPHIA-ANTIPOLIS CEDEX; FRANCE, RAN WG1, Spokane, USA, November    2018, discloses discussion on CSI enhancement in Re-16.-   3GPP DRAFT; R-1813357, 3^(RD) GENERATION PARTNERSHIP PROJECT (3GPP),    MOBILE COMPETENCE CENTER; 650, ROUTES DES LUCIOLES; F-06921    SOPHIA-ANTIPOLIS CEDEX; FRANCE, RAN WG1, Spokane, China, November    2018, discloses discussion on type II CSI overhead reduction.

SUMMARY

An object of embodiments herein is to provide methods and apparatuses inthe form a User Equipment (UE) and a radio base station or network nodeor gNB respectively for reducing (CSI) feedback overhead by employingefficient amplitude and phase quantization and reporting of coefficientsin a communications network that employs beamforming and/or MIMOoperation. Further, the present embodiments address the problem of howto efficiently quantize and report the transformed combiningcoefficients.

According to an aspect of embodiments herein, there is provided a methodperformed by a UE for reducing feedback overhead related to CSI in acommunications network employing MIMO operation:

-   -   decomposing each entry corresponding to a (i,j)-th combining        coefficient of a precoder matrix W₂ ^((r)), into at least two        coefficients, wherein r denotes a r-th transmission layer; said        (i,j)-th combining coefficient is associated with a i-th beam        and a j-th delay, and wherein each combining coefficient is        associated with an amplitude and a phase-information;    -   quantizing, separately, each of said at least two coefficients        with a least one bit, and    -   reporting information related to at least one phase value or at        least one amplitude value or at least one phase value and an        amplitude value of said quantized coefficient.

According to another aspect of embodiments herein, there is provided anapparatus in the form of UE for reducing feedback overhead in acommunications network, the UE comprising a processor and a memory, saidmemory containing instructions executable by said processor whereby saidUE is operative to perform any of the methods disclosed.

There is also provided a computer program comprising instructions whichwhen executed on at least one processor of the UE cause the at leastsaid one processor to carry out any of methods disclosed herein.

A carrier containing the computer program, wherein the carrier is one ofa computer readable storage medium; an electronic signal, optical signalor a radio signal.

There is also provided a method performed by a radio network node gNBfor reducing feedback overhead related to Channel State Information,CSI, in a communications network employing Multi Input Multi Output,MIMO operation, the method comprising: receiving, from a UE, a reportincluding information related to at least one phase value or at leastone amplitude value or at least one phase value and an amplitude valueeach quantized coefficient which is quantized with a least one bit by aUE;

wherein each entry corresponding to a (i,j)-th combining coefficient ofa precoder matrix W₂ ^((r)), is decomposed by the UE into at least twocoefficients, wherein r denotes a r-th transmission layer; said (i,j)-thcombining coefficient is associated with a i-th beam and a j-th delay,and wherein each combining coefficient is associated with an amplitudeand a phase-information;

According to another aspect of embodiments herein, there is alsoprovided a radio network node or gNB for reducing feedback overhead in acommunications network, the radio network node comprising a processorand a memory, said memory containing instructions executable by saidprocessor whereby said the radio network node is operative to performanyone of the methods disclosed herein.

There is also provided a computer program comprising instructions whichwhen executed on at least one processor of the radio network node, causethe at least said one processor to carry out any of the methodsdisclosed herein.

A carrier is also provided containing the computer program wherein thecarrier is one of a computer readable storage medium; an electronicsignal, optical signal or a radio signal.

Several advantages with the embodiments herein are presented in thedetailed part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments and advantages of the embodiments herein aredescribed in more detail with reference to attached drawings in which:

FIG. 1 depicts an example network scenario wherein embodiments hereinmay be employed.

FIG. 2 illustrates amplitude distribution of the combining coefficientsin matrix W₂ ^((r))

FIG. 3 Amplitude distribution of coefficients b_(i,j) for scheme 3according to an embodiment herein.

FIG. 4 illustrates a flowchart of a method performed by a UE accordingto some exemplary embodiments herein.

FIG. 5 is a block diagram depicting a UE according to exemplaryembodiments herein.

FIG. 6 is a block diagram depicting a radio network node according toexemplary embodiments herein.

DETAILED DESCRIPTION

In the following is presented a detailed description of the exemplaryembodiments in conjunction with the drawings, in several scenarios, toenable easier understanding of the solution(s) described herein.

As previously described, in 3GPP new radio system, two types ofcodebook, namely Type-1 and Type-2 codebook, have been standardized forthe CSI feedback in the support of advanced MIMO operation.

The present embodiments address the problem of how to efficientlyquantize and report the transformed combining coefficients in order toreduce CSI feedback overhead in a communications network employingbeamforming.

A. Quantization and Reporting of Complex Combining Coefficients ofMatrix W₂ ^((r)):

W₂ ^((r)) which was previously presented in equation (3) and repeatedbelow is the overall precoder which may be written as

F ^((r)) =F ₁ ^((r)) W ₂ ^((r)) K _(F) ^((r)) ^(T) ,  (3)

Each complex coefficient in W₂ ^((r)) in (3) is associated with aspecific delay (in the transformed domain) as each DFT/DCT basis vectormodels a linear phase increase over the subbands. W₂ ^((r)) containscomplex-combining coefficients. The values of N₁ and N₂ are designparameters and may be included in a configuration of an antenna array ata gNB, which antenna array may for example be dual-polarized, althoughthe embodiments herein are not restricted to dual-polarized antennaarrays.

An approach for quantizing the amplitude and phase values of thecoefficients in W₂ ^((r)), according to an exemplary embodiment, is toquantize directly each amplitude and phase value with N_(i) and N₂ bits,respectively.

For example, assuming that matrix W₂ ^((r)) contains UD coefficients,then UD(N₁+N₂) bits are required for reporting the amplitude and phaseinformation of W₂ ^((r)) to the gNB. However, as mentioned previously,each of the U beams are typically associated with only a set of delaysand not all D delays. Therefore, the matrix W₂ ^((r)) may be consideredas a sparse matrix where a large number of the coefficients are close tozero.

In the following description, the matrix W₂ ^((r))∈

^(U×D) in equation (3) may contain the complex combining coefficientsassociated with all (2L) spatial beams (i.e., U=2L), or only a subset ofspatial beams (e.g., U<2L), and/or all (V) delays/basis vectors (i.e.,D=V), or only a subset of delays/basis vectors (e.g., D<V).

1. Selection and Reporting of Non-Zero Coefficients by Using a Bitmap

In order to save feedback overhead for reporting a quantized version ofW₂ ^((r)), an approach according to an exemplary embodiment is tofeedback only the amplitude and phase-information of the non-zerocoefficients of matrix W₂ ^((r)) and to indicate by a bitmap the indicesof the reported coefficients. For example, the first bit in the bitmapmay be associated with the first coefficient, the second bit with thefirst coefficient of matrix W₂ ^((r)) and so on. When a bit in thebitmap is set to ‘1’ the corresponding coefficient (amplitude and/orphase) may be reported and otherwise not. In this way, the overhead forreporting the combining coefficients may be largely reduced; however,the number of feedback bits is not fixed and may vary for each reportinginstance. (see below how he number of feedback bits may be fixed).

2. Selection and Reporting of K Strongest Coefficients

In order to fix the number of feedback bits for reporting the combiningcoefficients, the receiver may be configured to feedback the amplitudeand/or phase values of the K strongest coefficients of matrix W₂ ^((r)),where the value of the parameter K is configurable by the gNB. The Kstrongest coefficients may be represented by the K entries having thehighest amplitude (or power) over the elements in W₂ ^((r)). When a bitin the bitmap is set to ‘1’, the UE may be configured to report thephase and/or amplitude values of the associated coefficient b_(i,j) tothe gNB. The bitmap may hence contain no more than K ‘1’s.

In order to increase the flexibility of selecting the coefficients andto improve the system performance, the receiver (e.g. a UE or anothergNB) may be configured to select K_(u) strongest coefficients perrow/beam out of matrix W₂ ^((r)), where the parameters K_(u) may beconfigurable by the gNB (transmitter). Note that the values of K_(u) maybe identical for a set of rows/beams of matrix W₂ ^((r)). In such acase, a single parameter R may be used to configure multipole parametersK_(d).

Similarly, the receiver may be configured to select K_(d) strongestcoefficients per column/delay out of matrix W₂ ^((r)), where theparameters K_(d) may be configurable by the gNB. Note that the values ofK_(d) may also be identical for a set of columns/delays of matrix W₂^((r)). In such a case, a single parameter O is used to configuremultiple parameters

3. Selection of Submatrices of F₂ ^((r)) for Reporting

According to an exemplary embodiment, to reduce the overhead forreporting the coefficients in W₂ ^((r)), the receiver may be configuredto report only the amplitude and/or phase information for a subset ofthe coefficients in W₂ ^((r)). The subset of coefficients in W₂ ^((r))may contain the combining coefficients associated with the “strongest”beams and/or “strongest” delays. In such a case, the rows and/or columnsof W₂ ^((r)) may be assumed to be ordered in such a way that thecombining-coefficients satisfy:

${\sum\limits_{\forall j}{\left\lbrack W_{2}^{(r)} \right\rbrack_{1,j}}} \geq {\sum\limits_{\forall j}{\left\lbrack W_{2}^{(r)} \right\rbrack_{2,j}}} \geq \cdots \geq {\sum\limits_{\forall j}{{\left\lbrack W_{2}^{(r)} \right\rbrack_{{IJ},j}}\mspace{14mu}{{and}/{or}}}}$∑_(∀i)[W₂^((r))]_(i, 1) ≥ ∑_(∀i)[W₂^((r))]_(i, 2) ≥ ⋯ ≥ ∑_(∀i)[W₂^((r))]_(i, D).

In an example, the receiver may be configured to report the amplitudeand/or phase information of the coefficients associated with the U′“strongest” beams. The receiver may then report the amplitude and/orphase information of the coefficients: {[W₂ ^((r))]_(i,j): i=1, . . . ,U′, ∀j}.

In another example, the receiver maybe configured to report theamplitude and/or phase information of the coefficients associated withthe D “strongest” delays. The receiver may report the amplitude and/orphase information of the coefficients: {[W₂ ^((r))]_(i,j): j=1, . . . ,D′, ∀i}.

In another example, the receiver may be configured to report theamplitude and/or phase information of the coefficients associated withthe U′ “strongest” beams and D “strongest” delays. The receiver mayreport the amplitude and/or phase information of the coefficients: {[W₂^((r))]_(i,j): i=1, . . . , U′, j=1, . . . , D′}.

In order to further significantly reduce the feedback overhead forreporting a quantized version of W₂ ^((r)), three decomposition andquantization schemes for W₂ ^((r)) are described according to someembodiments herein:

1. Scheme 1

The first scheme decomposes the (i,j)-th combining-coefficient of matrixW₂ ^((r)) associated with the i-th beam and j-th delay into twocoefficients, a_(i) and b_(i,j),

[W ₂ ^((r))]_(i,j) =a _(i) b _(i,j),

where b_(i,j) is the complex-valued normalized combining-coefficientassociated with the i-th beam and j-th delay, and a_(i) is a real-valuedcoefficient representing a common amplitude for the combiningcoefficients for all delays associated with the i-th beam. Note that thecalculation of the values a_(i) is implementation specific.

2. Scheme 2

The second scheme decomposes the (i,j)-th combining-coefficient ofmatrix W₂ ^((r)) associated with the i-th beam and j-th delay into twocoefficients, d_(j) and b_(i,j),

[W ₂ ^((r))]_(i,j) =d _(j) b _(i,j),

where b_(i,j) is the complex-valued normalized combining-coefficientassociated with the i-th beam and j-th delay, and d_(j) is a real-valuedcoefficient representing a common amplitude for the combiningcoefficients for all beams associated with the j-th delay. Note that thecalculation of the values d_(j) is implementation specific.

3. Scheme 3

The third scheme decomposes the (i,j)-th combining-coefficient of matrixW₂ ^((r)) associated with the i-th beam and j-th delay into threecoefficients, a and b_(i,j),

[W ₂ ^((r))]_(i,j) =a _(i) d _(j) b _(i,j),

where b_(i,j) is the complex-valued normalized combining-coefficientassociated with the i-th beam and j-th delay, d_(j) is a real-valuedcoefficient representing a common amplitude for the combiningcoefficients for all beams associated with the j-th delay, and a_(i) isa real-valued coefficient representing a common amplitude for thecombining coefficients for all delays associated with the i-th beam.Note that the calculation of the values a_(i) and d_(j) areimplementation specific.

The receiver may be configured to represent the combining coefficientsor only a set of the combining coefficients in W₂ ^((r)) by scheme 1,scheme 2, or scheme 3. Note that the proposed schemes can also becombined for representing the combining coefficients. For example, thereceive may be configured to represent a first set of combiningcoefficient of W₂ ^((r)) by scheme 1 or scheme 2, and a second set ofthe combining coefficients of W₂ ^((r)) by scheme 3.

4. Quantization of Coefficients a_(i), b_(i,j) and d_(j)

After the decomposition of each entry of W₂ ^((r)) into the coefficientsa_(i) and b_(i,j), or d_(j) and b_(i,j), or a_(i), d_(j) and b_(i,j),the coefficients are quantized separately in according with anembodiment. The main advantage of the above decomposition schemes isthat the amplitude values of b_(i,j) may be quantized with asignificantly lower number of bits than the amplitude values of thecombining-coefficients in W₂ ^((r)). Therefore, the feedback overheadfor reporting the amplitude values of the entries in W₂ ^((r)) advantagebe significantly reduced when applying one of the proposed decompositionschemes.

For example, the receiver may be configured to quantize the real-valuedcoefficients a_(i) (and/or d_(j)) equally with N_(a) (and/or N_(d))bits. Each complex-valued coefficient b_(i,j) may be quantized withN_(b,1) and N_(b,2) bits for the amplitude and phase, respectively,where N_(b,1) may be lower than B_(b,2).

Feedback Overhead Saving for Scheme 1:

For scheme 1, assuming there are UD combining coefficients contained inmatrix W₂ ^((r))∈

^(U×D), a total of UN_(a)+UD(N_(b,1)+N_(b,2)) bits are required forreporting the amplitude and phase information of the coefficients a_(i)and b_(i,j). In contrast, when directly quantizing the entries of matrixW₂ ^((r)) with N_(a) bits per amplitude and N_(a) bits per phase,2UDN_(a) bits are required for reporting the coefficients of W₂ ^((r)).Assuming that the phase values of b_(i,j) are quantized equally withN_(b,2)=N_(a) bits, the amount of feedback that may be saved by scheme 1may be given by U(D(N_(a)−N_(b,1))−N_(a)) bits. For typical values forthe number of beams (U), delays (D) and quantization bits (N_(a)) ofU=8, D=4, N_(a)=4, and N_(b,1)=2, a total of 32 bits may be saved forthe amplitude and phase reporting compared to the direct quantization ofthe entries in W₂ ^((r)).

Feedback Overhead Saving for Scheme 2:

For scheme 2, assuming again there are UD combining coefficientscontained in matrix W₂ ^((r))∈

^(U×D), a total of DN_(d)+UD(N_(b,1)+N_(b,2)) bits are required forreporting the amplitude and phase information of the coefficients d_(j)and b_(i,j). Assuming that the phase values of b_(i,j) are quantizedequally with N_(b,2)=N_(d) bits, the amount of feedback that may besaved by scheme 2 is given by D(U(N_(d)−N_(b,1))−N_(d)) bits. Fortypical values for the number of beams (U), delays (D) and quantizationbits (N_(d)) of U=8, D=4, N_(d)=4, and N_(b,1)=2, a total of 48 bitsmaybe saved for the amplitude and phase reporting compared to the directquantization of the entries in W₂ ^((r)).

Feedback Overhead Saving for Scheme 3:

For scheme 3, assuming again there are UD combining coefficientscontained in matrix W₂ ^((r))∈

^(U×D), a total of UN_(a)+DN_(d)+UD(N_(b,1)+N_(b,2)) bits are requiredfor reporting the amplitude and phase information of the coefficientsa_(i), d_(j) and b_(i,j). Assuming that the real-valued coefficientsa_(i) and d_(j) are equally quantized with N_(a)=N_(d) bits and thephase values of b_(i,j) are quantized with N_(b,2)=N_(a) bits, theamount of feedback that may be saved by scheme 3 is given byUD(N_(a)−N_(b,1))−(U+D)N_(a) bits. For typical values for the number ofbeams (U), delays (D) and quantization bits (N_(a)) of U=8, D=4,N_(a)=4, and N_(b,1)=1, a total of 48 bits may be saved for theamplitude and phase information reporting compared to the directquantization of the entries in W₂ ^((r)).

5. Selection and Reporting of Non-Zero Coefficients b_(i,j) by Using aBitmap

To reduce the overhead for reporting the coefficients in W₂ ^((r)), theUE may be configured to report only the phase values, only the amplitudevalues, or the amplitude and phase values of the quantized non-zerocoefficients b_(i,j). To indicate the indices of the quantized non-zerocoefficients b_(i,j), the receiver may be configured to report inaddition to the amplitude and/or phase information a bitmap, where eachbit in the bitmap is associated with a coefficient b_(i,j). For example,the first bit may be associated with coefficient b_(i,j), the second bitwith coefficient b_(1,2), etc. When a bit in the bitmap is set to one,the UE may report the phase and/or amplitude values of the associatedcoefficient b_(i,j) to the gNB. The bitmap may hence contain P “1”'s,where P corresponds to the number of non-zero coefficients b_(i,j).

6. Selection and Reporting of K Strongest Coefficients b_(i,j)

To reduce the overhead for reporting the coefficients in W₂ ^((r)) andto fix the number of feedback bits for CSI reporting, the UE may beconfigured to report only the phase values, only the amplitude values,or the amplitude and phase values of the K strongest coefficients ofmatrix W₂ ^((r)), where the value of the parameter K is configurable bythe gNB.

The K strongest coefficients may be represented by the K entries havingthe highest amplitude (or power) over the elements in W₂ ^((r)). Toindicate the indices of the K strongest elements, the receiver may beconfigured to report in addition to the K amplitude and/or phaseinformation a bitmap, where each bit in the bitmap is associated with acoefficient b_(i,j). For example, the first bit may be associated withcoefficient b_(i,j), the second bit with coefficient b_(1,2), etc. Whena bit in the bitmap is set to one, the UE may report the phase and/oramplitude values of the associated coefficient b_(i,j) to the gNB. Thebitmap may hence contain K “1”'s. In the case the number of non-zeroamplitude values of the quantized matrix W₂ ^((r)) is less than K, theUE may report only the amplitude and/or phase information with respectto the non-zero coefficients of the quantized matrix W₂ ^((r)). Thebitmap may then contain less than K “1”'s.

The amount of feedback required for reporting the amplitude and phaseinformation is given (for scheme 3) by UN_(a)+DN_(d)+K(N_(b,1)+N_(b,2))bits for the amplitude and phase information of W₂ ^((r)) and UD bitsfor the bitmap. Hence, a total of (UD K)(N_(b,1)+N_(b,2)) UD bits may besaved compared to the case of reporting the amplitude and phaseinformation of all coefficients in W₂ ^((r)) to the gNB.

In order to increase the flexibility of selecting the coefficients andto improve the system performance, the receiver may be configured toselect K_(u) strongest coefficients per row/beam out of matrix W₂^((r)), where the parameters K_(u) may be configurable by the gNB. Notethat the values of K_(u) may be identical for a set of rows/beams ofmatrix W₂ ^((r)). In such a case, a single parameter R may be used toconfigure multipole parameters K_(d).

Similarly, the receiver may be configured to select K_(d) strongestcoefficients per column/delay out of matrix W₂ ^((r)), where theparameters K_(d) may be configurable by the gNB. Note that the values ofK_(d) can also be identical for a set of columns/delays of matrix W₂^((r)). In such a case, a single parameter O is used to configuremultiple parameters K_(d).

7. Reporting of Phase-Only Information of Matrix B without BitmapIndication and 1-Bit Amplitude Quantization

FIG. 2 and FIG. 3 show the amplitude distribution of the combiningcoefficients in matrix W₂ ^((r)) and the amplitude distribution ofcoefficients b_(i,j) when applying the proposed third decompositionscheme according to previously described embodiment.

As shown, when applying the proposed third decomposition scheme, thecoefficients b_(i,j) may be efficiently represented by only twoquantization levels in contrast to combining coefficients in matrix W₂^((r)). The amplitude information of the coefficients b_(i,j) cantherefore be quantized using only one bit for the amplitude values. Thereceiver may therefore to be configured with N_(b,1)=1 and eachamplitude value may be represented by two quantization levels “a” and“b”, where for example “a” and/or “b” are given by “a=0” and “b=1”. Thebits in the bitmap then directly correspond to the two quantizationlevels of the amplitude values of the coefficients b_(i,j) and anadditional report of the amplitude values of b_(i,j) is not required. AsN_(b,1)=1, the amount of feedback for reporting the phase values isdrastically reduced as well, since a reporting of the phase valuesassociated with zero amplitude coefficients is not required. The samequantization levels may also be used for scheme 1 and scheme 2 forquantizing coefficients b_(i,j).

Note that the above also holds for scheme 1 and scheme 2, i.e. thereceiver may be configured with N_(b,1)=1 each amplitude value may berepresented by two quantization levels “a” and “b”, where for example“a” and/or “b” are given by “a=0” and “b=1”.

8. Different Quantization Levels for the Phase Values of b_(i,j)

To further reduce the overhead for reporting the phase information ofthe coefficients b_(i,j), the receiver may be configured to applydifferent quantization levels for the phase values of the coefficientsb_(i,j). For example, the receiver may configured to use N_(b,2)′ bitsfor the phase values associated with the non-zero coefficients and theU′ strongest beams and N_(b,2)″ bits for the phase values associatedwith the non-zero coefficients and the remaining beams, whereN_(b,2)′>N_(b,2)″.

B. Reporting of the Indices Associated with the Selected DFT/DCTVectors:

In addition to the reporting of quantized coefficients of matrix W₂^((r)), the following describes an approach for efficiently reportingthe indices of the DFT/DCT vectors associated with the complex combiningcoefficients of matrix K_(F) ^((r)) according with an exemplaryembodiment herein. The DFT/DCT vectors are selected from a set ofpredefined DFT/DCT basis vectors, where each DFT/DCT basis vector isassociated with an index. For example, when there are S DFT/DCT basisvectors, the first DFT/DCT basis vector is associated with a first index(“1”), the second DFT/DCT basis vector is associated with a second index(“1”), and the last DFT/DCT basis vector is associated with the index(“S”). When reporting D selected DFT/DCT basis vectors, D┌log₂(S)┐feedback bits are required.

Instead of directly reporting the indices of the DFT/DCT basis vectors,the receiver may be configured to report a bitmap, where each bit in thebitmap is associated with an index “d” from the set of basis vectors.

For example, the first bit may be associated with index 1, the secondbit with index 2, etc. A “1” in a bitmap at position “d” indicates thenthe selection of the DFT/DCT vector associated with the index “d”.

As an example, when the number of subbands S=13 and D=6, the amount offeedback required to report the indices of the selected DFT/DCT vectorsis given by D┌log₂(S)┐=24, where in contrast only S=13 bits are requiredwhen using a bitmap.

According to an embodiment, when the reported bitmap is comprised of a“1” at position “1”, then the amplitude and phase values of the leadingbeam must be considered as follows:

A “1” at position 1 of the bitmap indicates that the amplitude and phaseof the combining coefficient of the leading beam associated with index“1” are given by 1 and 0, respectively, and are not reported. Theamplitude and phase values of the remaining combining coefficients ofthe leading beam associated with other indices are given by 0 and 0,respectively, and are not reported. The amplitude and phase associatedwith the leading beam are known at the gNB.

As initially described, the proposed solutions are suitable for the 3GPPRel-15 framework. Below is presented modifications (1)-(7) that areherein suggested by the inventors to the Rel. 15 framework according tosome embodiments herein

-   -   (1) The number of leading beams (B) for which the amplitude        values of b_(i,j) shall be reported to the gNB is given by B=2L        or 2L−1 for the proposed CSI reporting using DFT/DCT        transformation instead of B=4, 4, and 6 for L=2, 3, and 4 as in        current Rel. 15, where L is the number of spatial beams        configured.    -   (2) All quantized amplitude and phase values of b_(i,j)        associated with the first leading beam are not reported to the        gNB.    -   (3) When N_(a)=3, the amplitude set for quantizing a_(i) is        given        -   {1, √{square root over (0.5)}, √{square root over (0.25)},            √{square root over (0.125)}, √{square root over (0.0625)},            √{square root over (0.0313)}, √{square root over (0.0156)},            0}    -   (4) When N_(d)=3, the amplitude set for quantizing d_(i) is        given uniform        -   {1, √{square root over (0.5)}, √{square root over (0.25)},            √{square root over (0.125)}, √{square root over (0.0625)},            √{square root over (0.0313)}, √{square root over (0.0156)},            0}    -   (5) When N_(d)=2, the amplitude set for quantizing d_(i) is        given        -   {, √{square root over (0.5)}, √{square root over (0.25)}, 0}    -   (6) The amplitude set for quantization of b_(i,j) is given {0,        1}.    -   (7) The phase set for quantizing b_(i,j) is given by a 8PSK, or        16PSK constellation.

It should be mentioned that differently to equation (3) above, one mayintroduce in the following a new transformation/decomposition of the U×Smatrix F₂ ^((r)) that may be combined with the above mentioned threedecomposition/quantization schemes. Compared to equation (3), thefollowing transformation/decomposition reduces further the overhead ofreporting the combining coefficients when combined with theabove-mentioned three decomposition/quantization schemes. The frequencydomain combining coefficient matrix F₂ ^((r)) is decomposed into threematrices,

F ₂ ^((r)) =A ^((r)) F ₂ ^((r)) B ^((r)),

where

-   -   A^((r)) is a real-valued U×U diagonal matrix containing U        “wideband” amplitude coefficients of the respective rows/beams        of matrix F₂ ^((r)),    -   F ₂ ^((r)) is a complex-valued U×S matrix containing US        “subband” combining coefficients of the U beams and S subbands,        and    -   B^((r)) is a real-valued S×S diagonal matrix containing S values        on its diagonal.

The matrix A^((r)) contains the values of the “average” amplitudes ofthe rows/beams of the combining coefficients of matrix F₂ ^((r)). Thematrix B^((r)) is a normalization matrix that forces the S combiningcoefficients in the row of matrix F ₂ ^((r)) which are associated withthe leading beam to be “1”. Note that the leading beam is associatedwith the highest “wideband” amplitude coefficient of matrix A^((r)). Byconsidering now a transformation on the subband coefficient matrix FP,the frequency domain combining coefficients F₂ ^((r)) may be written as.

$\begin{matrix}{{F_{2}^{(r)} = {A^{(r)}{\overset{\_}{W}}_{2}^{(r)}C^{(r)}K_{F}^{{(r)}^{T}}B^{(r)}}},} \\{= {W_{2}^{(r)}K_{F}^{{(r)}^{T}}B^{(r)}}}\end{matrix}$

where W₂ ^((r))=A^((r))>W ₂ ^((r))C^((r)) with W ₂ ^((r))∈

^(2L×V) containing 2L×V complex-combining coefficients associated withthe V basis vectors in matrix K_(F) ^((r))∈

^(V×S) (see equation (3)) and C^((r)) is a V×V diagonal matrixcontaining the “common” amplitude values of the V basis vectors.

The quantization and reporting of W₂ ^((r)) is as previously described

For Reporting of B^((r)) the receiver (or e.g. the UE) may be configuredto report or not to report the S coefficients of the diagonal matrixB^((r)) using N_(B) bits per coefficients. When the receiver isconfigured not to report the S coefficients, the transmitter (e.g. thegNB) assumes that the matrix B^((r)) is given by an identity matrix whenreconstructing the precoder matrix. Note that the coefficients in matrixB^((r)) may be represented by only two quantization levels “a” and “b”,where for example “a” and/or “b” are given by “a=√{square root over(0.5)},” and “b=1”.

It should be mentioned that all parameters and denoted values may takeany suitable values and some or all are design parameters which whenused achieve the technical effects and advantages of the embodimentsdescribed in this disclosure.

Referring to FIG. 4, there is illustrated a flowchart of a methodperformed by a UE 500 for reducing feedback overhead related to CSI in acommunications network employing MIMO operation, the method comprises:

(401) decomposing each entry corresponding to a (i,j)-th combiningcoefficient of a precoder matrix, W₂ ^((r)), into at least twocoefficients, wherein r denotes a r-th transmission layer; said (i,j)-thcombining coefficient is associated with a i-th beam and a j-th delay,and wherein each combining coefficient is associated with an amplitudeand a phase-information;

-   -   (402) quantizing, separately, each of said at least two        coefficients with a least one bit, and    -   (403) reporting information related to at least one phase value        or at least one amplitude value or at least one phase value and        an amplitude value of said quantized coefficient.

The method performed by the UE, according to some embodiments herein ispresented in the subject-matter of each of the claims listed below.

In order to perform the previously described process or method stepsrelated to the UE, embodiments herein include a UE for reducing feedbackoverhead in a communications network. As shown in FIG. 5, the UE 500comprises a processor 510 or processing circuit or a processing moduleor a processor or means 510; a receiver circuit or receiver module 540;a transmitter circuit or transmitter module 550; a memory module 520 atransceiver circuit or transceiver module 530 which may include thetransmitter circuit 550 and the receiver circuit 540. The UE 500 furthercomprises an antenna system 560 which includes antenna circuitry fortransmitting and receiving signals to/from at least a radio network nodeor gNB. The antenna system may employ beamforming as previouslydescribed.

The UE 500 may operate in any radio access technology including 2G, 3G,4G or LTE, LTE-A, 5G, WLAN, and WiMax etc. that support beamformingtechnology.

The processing module/circuit 510 includes a processor, microprocessor,an application specific integrated circuit (ASIC), field programmablegate array (FPGA), or the like, and may be referred to as the “processor510.” The processor 510 controls the operation of the network node 500and its components. Memory (circuit or module) 520 includes a randomaccess memory (RAM), a read only memory (ROM), and/or another type ofmemory to store data and instructions that may be used by processor 510.In general, it will be understood that the UE 500 in one or moreembodiments includes fixed or programmed circuitry that is configured tocarry out the operations in any of the embodiments disclosed herein.

In at least one such example, the UE 500 includes a microprocessor,microcontroller, DSP, ASIC, FPGA, or other processing circuitry that isconfigured to execute computer program instructions from a computerprogram stored in a non-transitory computer-readable medium that is in,or is accessible to the processing circuitry. Here, “non-transitory”does not necessarily mean permanent or unchanging storage, and mayinclude storage in working or volatile memory, but the term does connotestorage of at least some persistence. The execution of the programinstructions specially adapts or configures the processing circuitry tocarry out the operations disclosed herein including any one of theclaims listed below.

Further, it will be appreciated that the UE 500 may comprise additionalcomponents not shown in FIG. 5.

As previously presented, the UE 500 is operative to: decompose eachentry corresponding to a (i,j)-th combining coefficient of a precodermatrix W₂ ^((r)), into at least two coefficients, wherein r denotes ar-th transmission layer; said (i,j)-th combining coefficient isassociated with a i-th beam and a j-th delay, and wherein each combiningcoefficient is associated with an amplitude and a phase-information;quantize, separately, each of said at least two coefficients with aleast one bit, and reporting information related to at least one phasevalue or at least one amplitude value or at least one phase value and anamplitude value of said quantized coefficient.

The UE is configured to decompose using a first scheme, the (i,j)-thcombining-coefficient of said matrix W₂ ^((r)) associated with the i-thbeam and j-th delay into two coefficients, a_(i) and b_(i,j),

[W ₂ ^((r))]_(i,j) =a _(i) b _(i,j),

where b_(i,j) is complex-valued normalized combining-coefficientassociated with the i-th beam and j-th delay, and a_(i) is a real-valuedcoefficient representing a common amplitude for the combiningcoefficients for all delays associated with the i-th beam.

The UE (500) is configured to decompose using a second scheme, the(i,j)-th combining-coefficient of matrix W₂ ^((r)) associated with thei-th beam and j-th delay into two coefficients, d_(j) and b_(i,j),

[W ₂ ^((r))]_(i,j) =d _(j) b _(i,j),

where b_(i,j) is the complex-valued normalized combining-coefficientassociated with the i-th beam and j-th delay, and d_(j) is a real-valuedcoefficient representing a common amplitude for the combiningcoefficients for all beams associated with the j-th delay.

The UE (500) is configured to decompose, using a third scheme the(i,j)-th combining-coefficient of matrix W₂ ^((r)) associated with thei-th beam and j-th delay into three coefficients, a_(j), d_(j) andb_(i,j),

[W ₂ ^((r))]_(i,j) =a _(i) d _(j) b _(i,j),

where b_(i,j) is the complex-valued normalized combining-coefficientassociated with the i-th beam and j-th delay, d_(j) is a real-valuedcoefficient representing a common amplitude for the combiningcoefficients for all beams associated with the j-th delay, and a_(i) isa real-valued coefficient representing a common amplitude for thecombining coefficients for all delays associated with the i-th beam.

The UE (500) is configured to represent the combining coefficients oronly a set of the combining coefficients in W₂ ^((r)) by the firstscheme, the second scheme, or the third scheme.

Additional details relating to the functionality or actions performed bythe UE have already been disclosed (see method steps performed by theUE).

There is also provided a computer program comprising instructions whichwhen executed on at least one processor 510 of the UE, cause theprocessor 510 to carry out the method according to any one of the claimslisted below.

There is also provided a method performed by a radio base station or aradio network node or a gNB 700 according to some exemplary embodiments:

The method for reducing feedback overhead related to CSI in thecommunications network employing MIMO operation comprises:

(601) receiving, from a UE 500 a report including information related toat least one phase value or at least one amplitude value or at least onephase value and an amplitude value each quantized coefficient which isquantized with a least one bit by a UE 500; wherein each entrycorresponding to a (i, j)-th combining coefficient of a precoder matrixW₂ ^((r)), is decomposed by the UE 500 into at least two coefficients,wherein r denotes a r-th transmission layer; said (i,j)-th combiningcoefficient is associated with a i-th beam and a j-th delay, and whereineach combining coefficient is associated with an amplitude and aphase-information;

The method comprises configuring the UE to feedback the amplitude and/orphase values of K strongest coefficients of matrix W₂ ^((r)), whereinthe value of K is configurable by radio network node (700) or the gNB.

The method comprises configuring the UE comprises configuring the UE toselect K_(u) strongest coefficients per row/beam out of matrix W₂^((r)), where the parameters K_(u) is configurable by the radio basestation or gNB.

The method comprises configuring the UE to select K_(d) strongestcoefficients per column/delay out of matrix W₂ ^((r)), where theparameters K_(d) is configurable by the radio base station (700) or gNB.

The method comprises configuring the UE to report only the amplitudeand/or phase information for a subset of the coefficients in W₂ ^((r)).

The method comprises configuring the UE the UE to represent thecombining coefficients or only a set of the combining coefficients in W₂^((r)) by the first scheme of claim 2 or the second scheme of claim 3 orthe third scheme of claim 4.

The method comprises configuring the UE to quantize the real-valuedcoefficients a_(i) (and/or d_(j)) equally with N_(a) (and/or N_(d))bits, wherein a_(i) is a real-valued coefficient representing a commonamplitude for the combining coefficients for all delays associated withthe i-th beam.

The method comprises configuring the UE comprises configuring the UE toreport only the phase values, only the amplitude values, or theamplitude and phase values of the quantized non-zero coefficientsb_(i,j).

The method comprises configuring the UE configuring the UE to report abitmap, where each bit in the bitmap is associated with an index “d”from a set of DFT/DCT basis vectors.

Additional functions performed by the radio network node 700 has alreadybeen disclosed and need no repetition.

In order to perform the previously described process or method stepsrelated to the radio network node, some embodiments herein include aradio network node 700 for reducing feedback overhead in acommunications network.

As shown in FIG. 7, the radio network node 700 comprises a processor 710or processing circuit or a processing module or a processor or means710; a receiver circuit or receiver module 740; a transmitter circuit ortransmitter module 770; a memory module 720 a transceiver circuit ortransceiver module 730 which may include the transmitter circuit 770 andthe receiver circuit 740. The radio network node 700 further comprisesan antenna system 760 which includes antenna circuitry for transmittingand receiving signals to/from at least network nodes and other UEs etc.The antenna system employs beamforming as previously described.

The radio network node 700 may operate in any radio access technologyincluding 2G, 3G, 4G or LTE, LTE-A, 5G, WLAN, and WiMax etc. thatsupport beamforming technology.

The processing module/circuit 710 includes a processor, microprocessor,an application specific integrated circuit (ASIC), field programmablegate array (FPGA), or the like, and may be referred to as the “processor710.” The processor 710 controls the operation of the UE 700 and itscomponents. Memory (circuit or module) 720 includes a random accessmemory (RAM), a read only memory (ROM), and/or another type of memory tostore data and instructions that may be used by processor 710. Ingeneral, it will be understood that the radio network node 700 in one ormore embodiments includes fixed or programmed circuitry that isconfigured to carry out the operations in any of the embodimentsdisclosed herein.

In at least one such example, the radio network node 700 includes amicroprocessor, microcontroller, DSP, ASIC, FPGA, or other processingcircuitry that is configured to execute computer program instructionsfrom a computer program stored in a non-transitory computer-readablemedium that is in, or is accessible to the processing circuitry. Here,“non-transitory” does not necessarily mean permanent or unchangingstorage, and may include storage in working or volatile memory, but theterm does connote storage of at least some persistence. The execution ofthe program instructions specially adapts or configures the processingcircuitry to carry out any the operations disclosed herein. Further, itwill be appreciated that the radio network node 700 may compriseadditional components not shown in FIG. 7. For reducing feedbackoverhead related to Channel State Information, CSI, in a communicationsnetwork employing Multi Input Multi Output, MIMO operation, the gNB isoperative to perform any one of the claims listed below.

Additional functions performed by the radio network node 700 havealready been disclosed and need not be repeated again.

There is also provided a computer program comprising instructions whichwhen executed on at least one processor 710 of the radio network node700, cause the at least said one processor 710 to carry out the methodaccording to any of the claims listed below.

A carrier containing the computer program is also provided, wherein thecarrier is one of a computer readable storage medium; an electronicsignal, optical signal or a radio signal.

As evident from the detailed description presented above, severaladvantages are achieved by the disclosed embodiments.

Throughout this disclosure, the word “comprise” or “comprising” has beenused in a non-limiting sense, i.e. meaning “consist at least of”.Although specific terms may be employed herein, they are used in ageneric and descriptive sense only and not for purposes of limitation.The embodiments herein may be applied in any wireless systems includingGSM, 3G or WCDMA, LTE or 4G, LTE-A (or LTE-Advanced), 5G, WiMAX, WiFi,satellite communications, TV broadcasting etc. that may employbeamforming technology.

REFERENCES

-   [1] Samsung, “Revised WID: Enhancements on MIMO for NR”, RP-182067,    3GPP RAN#81, Gold Coast, Australia, Sep. 10-13, 2018.-   [2] Fraunhofer I I S, Fraunhofer H H I, “Enhancements on Type II CSI    reporting scheme,” R1-1806124, Busan, South Korea, May 21-25, 2018.-   [3] Fraunhofer I I S, Fraunhofer H H I, “Enhancements on Type-II CSI    reporting,” R1-1811088, Chengdu, China, Oct. 8-12, 2018.-   [4] Fraunhofer I I S, Fraunhofer H H I, “Space-delay versus sub-band    precoding for mmWave channels,” R1-1800597, Vancouver, Canada, Jan.    22-26, 2018.-   [5] Ericsson, “On CSI enhancements for MU-MIMO support,” R1-1811193,    Chengdu, China, Oct. 8-12, 2018.-   [6] Huawei, HiSilicon, “Discussion on CSI enhancement for MU-MIMO,”    R1-1810103, Chengdu, China, Oct. 8-12, 2018.-   [7] Fraunhofer I I S, Fraunhofer H H I, “Enhancements on Type II CSI    reporting scheme,” R1-1813130, Spokane, USA, Nov. 12-16, 2018.-   [8] 3GPP TS 38.214 V15.3.0, “3GPP; TSG RAN; NR; Physical layer    procedures for data (Release 15)”, September 2018.-   [9] Nokia, Nokia Shanghai Bell, “CSI feedback overhead reduction for    MU-MIMO enhancements,” R1-1813488, Nov. 12-16, 2018.

1. A method performed by a User Equipment, UE, for reducing feedbackoverhead related to Channel State Information, CSI, in a communicationsnetwork employing Multi Input Multi Output, MIMO operation, the methodcomprising: decomposing each entry corresponding to a (i,j)-th combiningcoefficient of a precoder matrix F^((r)), into at least twocoefficients, wherein r denotes a r-th transmission layer; said (i,j)-thcombining coefficient is associated with a i-th beam and a j-th delay,and wherein each combining coefficient is associated with an amplitudeand a phase-information; wherein the precoder matrix F^((r)) is aproduct of a wideband first-stage precoder matrix F₁ ^((r)) containing2L spatial beams identical for all subbands, said matrix W₂ ^((r)), anda matrix K_(F) ^((r)) composed of a number of basis vectors used toperform a compression in a frequency domain; wherein the decomposed(i,j)-th combining-coefficient of said matrix W₂ ^((r)) is given by:[W ₂ ^((r))]_(i,j) =a _(i) b _(i,j),  where b_(i,j) is a complex-valuednormalized combining coefficient associated with the i-th beam and j-thdelay, and a_(i) is a real-valued coefficient representing a commonamplitude for the combining coefficients for all delays associated withthe i-th beam; quantizing, separately, each of said at least twocoefficients with a least one bit, and reporting information related toat least one phase value or at least one amplitude value or at least onephase value and an amplitude value of said quantized coefficient. 2-6.(canceled)
 7. The method according to claim 1, further comprisingquantizing the real-valued coefficients a_(i) equally with N_(a) bits,wherein each complex-valued coefficient b_(i,j) is quantized withN_(b,1) and N_(b,2) bits for the amplitude and phase, respectively,where N_(b,1) may be lower than N_(b,2).
 8. The method according toclaim 1, further comprising 2 quantizing the entries of matrix W₂ ^((r))with N_(a) bits per amplitude and N_(a) bits per phase, and using2UDN_(a) bits for reporting the coefficients of W₂ ^((r)), wherein UD isthe number of combining coefficients in matrix W₂ ^((r)). 9-10.(canceled)
 11. The method according to claim 1, further comprisingreporting only the phase values, only the amplitude values, or theamplitude and phase values of K strongest coefficients of matrix W₂^((r)), where the value of the parameter K is configurable by a radionetwork node or gNB.
 12. The method according to claim 11, furthercomprising selecting K_(u) strongest coefficients per row/beam out ofmatrix W₂ ^((r)), where the parameters K_(u) is configurable by theradio network node or gNB.
 13. The method according to claim 11, furthercomprising selecting K_(d) strongest coefficients per column/delay outof matrix W₂ ^((r)), where the parameters K_(d) is configurable by theradio network node gNB.
 14. (canceled)
 15. The method according to claim1, further comprising, the UE is configurable by the radio base stationor gNB with N_(b,1)=1 and representing each amplitude value by twoquantization levels “a” and “b”, where for example “a” and/or “b” aregiven by “a=0” and “b=1”.
 16. The method according to claim 1, furthercomprising applying different quantization levels for the phase valuesof the coefficients b_(i,j).
 17. The method according to claim 16,further comprising using N_(b,2)′ bits for the phase values associatedwith non-zero coefficients and the U′ strongest beams and N_(b,2)″ bitsfor the phase values associated with the non-zero coefficients and theremaining beams, where N_(b,2)′>N_(b,2)″.
 18. The method according toclaim 1, further comprising reporting indices of Discrete FourierTransform/Discrete Cosine Transform, DFT/DCT vectors associated with thecomplex combining coefficients of matrix K_(F) ^((r)).
 19. The methodaccording to claim 18, further comprising selecting the DFT/DCT vectorsfrom a set of predefined DFT/DCT basis vectors, where each DFT/DCT basisvector is associated with an index.
 20. (canceled)
 21. The methodaccording to claim 19, further comprising reporting a bitmap, where eachbit in the bitmap is associated with an index “d” from the set ofDiscrete Fourier Transform/Discrete Cosine Transform, DFT/DCT basisvectors associated with the complex combining coefficients of matrixK_(F) ^((r)).
 22. The method according to claim 21, further comprisingwhen bitmap consists of a “1” at position “1”, then the amplitude andphase values of the leading beam are considered as follows: A “1” atposition 1 of the bitmap indicates that the amplitude and phase of thecombining coefficient of the leading beam associated with index “1” aregiven by 1 and 0, respectively, and are not reported; and the amplitudeand phase values of the remaining combining coefficients of the leadingbeam associated with other indices are given by 0 and 0, respectively,and are not reported.
 23. The method according to claim 1, wherein thenumber of leading beams, B, for which the amplitude values of b_(i,j)shall be reported to a radio base station or gNB is given by B=2L or2L−1 for reporting using a DFT/DCT transformation, where L is a numberof spatial beams configured.
 24. The method according to claim 1,further comprising not reporting to a radio base station or a gNB thequantized amplitude and phase values of b_(i,j) associated with thefirst leading beam.
 25. The method according to claim 8, wherein whenN_(a)=3, the amplitude set for quantizing a_(i) is given {1, √{squareroot over (0.5)}, √{square root over (0.25)}, √{square root over(0.125)}, √{square root over (0.0625)}, √{square root over (0.0313)},√{square root over (0.0156)}, 0}. 26-27. (canceled)
 28. The methodaccording to claim 1, wherein the amplitude set for quantization ofb_(i,j) is given by {0, 1}.
 29. The method according to claim 1, whereinthe phase set for quantizing b_(i,j) is given by a 8PSK, Phase ShiftKeying, constellation or a 16PSK constellation.
 30. A User Equipment forreducing feedback overhead related to Channel State Information, CSI, ina communications network employing Multi Input Multi Output, MIMOoperation, the UE comprising a processor and a memory containinginstructions executable by said processor whereby the UE is operativeto: decompose each entry corresponding to a (i,j)-th combiningcoefficient of a precoder matrix F^((r)), into at least twocoefficients, wherein r denotes a r-th transmission layer; said (i,j)-thcombining coefficient is associated with a i-th beam and a j-th delay,and wherein each combining coefficient is associated with an amplitudeand a phase-information; wherein the precoder matrix F^((r)) is aproduct of a wideband first-stage precoder matrix F₁ ^((r)) containing2L spatial beams identical for all subbands, said matrix W₂ ^((r)), anda matrix K_(F) ^((r)) composed of a number of basis vectors used toperform a compression in a frequency domain; wherein the decomposed(i,j)-th combining-coefficient of said matrix W₂ ^((r)) is given by:[W ₂ ^((r))]_(i,j) =a _(i) b _(i,j),  where b_(i,j) is a complex-valuednormalized combining coefficient associated with the i-th beam and j-thdelay, and a_(i) is a real-valued coefficient representing a commonamplitude for the combining coefficients for all delays associated withthe i-th beam; quantize, separately, each of said at least twocoefficients with a least one bit, and report information related to atleast one phase value or at least one amplitude value or at least onephase value and an amplitude value of said quantized coefficient.
 31. Amethod performed by a radio network node or gNB for reducing feedbackoverhead related to Channel State Information, CSI, in a communicationsnetwork employing Multi Input Multi Output, MIMO operation, the methodcomprising: receiving, from a UE, a report including information relatedto at least one phase value or at least one amplitude value or at leastone phase value and an amplitude value of each quantized coefficientwhich is quantized with a least one bit by a UE; wherein each entrycorresponding to a (i,j)-th combining coefficient of a precoder matrixW₂ ^((r)), of a precoder matrix F^((r)), is decomposed by the UE into atleast two coefficients, wherein r denotes a r-th transmission layer;said (i,j)-th combining coefficient is associated with a i-th beam and aj-th delay, and wherein each combining coefficient is associated with anamplitude and a phase-information; wherein the precoder matrix F^((r))is a product of a wideband first-stage precoder matrix F₁ ^((r))containing 2L spatial beams identical for all subbands, said matrix W₂^((r)), and a matrix K_(F) ^((r)) composed of a number of basis vectorsused to perform a compression in a frequency domain; wherein thedecomposed (i,j)-th combining-coefficient of said matrix W₂ ^((r)) isgiven by:[W ₂ ^((r))]_(i,j) =a _(i) b _(i,j), where b_(i,j) is a complex-valuednormalized combining-coefficient associated with the i-th beam and j-thdelay, and a_(i) is a real-valued coefficient representing a commonamplitude for the combining coefficients for all delays associated withthe i-th beam.
 32. The method according to claim 31, further comprisingconfiguring the UE to feedback the amplitude and/or phase values of Kstrongest coefficients of matrix W₂ ^((r)), wherein the value of K isconfigurable by radio network node or the gNB.
 33. The method accordingto claim 31, further comprising configuring the UE to select K_(u)strongest coefficients per row/beam out of matrix W₂ ^((r)), where theparameters K_(u) is configurable by the radio base station or the gNB.34. The method according to claim 31, further comprising configuring theUE to select K_(d) strongest coefficients per column/delay out of matrixW₂ ^((r)), where the parameter K_(d) is configurable by the radio basestation or gNB.
 35. The method according to claim 31, further comprisingconfiguring the UE to report only the amplitude and/or phase informationfor a subset of the coefficients in W₂ ^((r)).
 36. (canceled)
 37. Themethod according to claim 31, further comprising configuring the UE toquantize the real-valued coefficients a_(i) (and/or d_(j)) equally withN_(a)(and/or N_(d)) bits, wherein a_(i) is a real-valued coefficientrepresenting a common amplitude for the combining coefficients for alldelays associated with the i-th beam.
 38. The method according to claim31, further comprising configuring the UE to report only the phasevalues, only the amplitude values, or the amplitude and phase values ofthe quantized non-zero coefficients b_(i,j).
 39. The method according toclaim 31, further comprising configuring the UE to report a bitmap,where each bit in the bitmap is associated with an index “d” from a setof DFT/DCT basis vectors.
 40. A radio network node or a gNB for reducingfeedback overhead related to Channel State Information, CSI, in acommunications network employing Multi Input Multi Output, MIMOoperation, the gNB comprising a processor and a memory containinginstructions executable by said processor whereby the gNB is operativeto: receive, from a UE, a report including information related to atleast one phase value or at least one amplitude value or at least onephase value and an amplitude value of each quantized coefficient whichis quantized with a least one bit by a UE; wherein each entrycorresponding to a (i,j)-th combining coefficient of a precoder matrixW₂ ^((r)), of a precoder matrix F^((r)), is decomposed by the UE into atleast two coefficients, wherein r denotes a r-th transmission layer;said (i,j)-th combining coefficient is associated with a i-th beam and aj-th delay, and wherein each combining coefficient is associated with anamplitude and a phase-information; wherein the precoder matrix F^((r))is a product of a wideband first-stage precoder matrix F₁ ^((r))containing 2L spatial beams identical for all subbands, said matrix W₂^((r)), and a matrix K_(F) ^((r)) composed of a number of basis vectorsused to perform a compression in a frequency domain; wherein thedecomposed (i,j)-th combining-coefficient of said matrix W₂ ^((r)) isgiven by:[W ₂ ^((r))]_(i,j) =a _(i) b _(i,j),  where b_(i,j) is a complex-valuednormalized combining-coefficient associated with the i-th beam and j-thdelay, and a_(i) is a real-valued coefficient representing a commonamplitude for the combining coefficients for all delays associated withthe i-th beam.
 41. The method according to claim 1, further comprisingreporting a bitmap wherein each bit in the bitmap is associated withsaid coefficient b_(i,j) and when a bit is set to one, the phase andamplitude values associated with the coefficient b_(i,j) are reported.42. The method according to claim 41, wherein the bitmap comprises K orless than K ‘1’s.
 43. The method according to claim 1, furthercomprising reporting the amplitude and phase information with respect tothe non-zero coefficients of the quantized matrix W₂ ^((r)) when thenumber of non-zero amplitude values of the quantized matrix W₂ ^((r)) isless than K.
 44. The method according to claim 1, further comprisingreporting information related to only the phase values, only theamplitude values, or only the amplitude and the phase values of thequantized non-zero coefficients b_(i,j).
 45. The method according toclaim 31, further comprising receiving from a UE a bitmap wherein eachbit in the bitmap is associated with said coefficient b_(i,j) and when abit is set to one, the phase and amplitude values associated with thecoefficient b_(i,j) are reported.
 46. The method according to claim 44,wherein the bitmap comprises K or less than K ‘1’s.